Significance: Families of children with ASD experience heightened risk due to elevated rates of clinically-significant parenting stress and child externalizing behavior problems. Parenting stress is a robust predictor of subsquent externalizing challenges in children with ASD. Nonetheless, few evidenced-based treatments exist for reducing parenting stress in these families. The mechanisms through which parenting stress influence child externalizing problems are also unclear, although preliminary evidence suggests the potential role of negative parenting behaviors. The proposed study comprehensively addresses these considerations by testing the efficacy of Mindfulness-Based Stress Reduction (MBSR) as an intervention to reduce parenting stress, lessen parental reactivity and negativity, and decrease child externalizing behaviors. Innovation: MBSR is particularly well-suited for parents of children with ASD given the intervention emphasis on teaching participants to manage reactivity in the context of persistent stress. However, the efficacy of MBSR has yet to be established for this population. The present investigation extends preliminary investigations of mindfulness approaches by: 1) conducting a stringent test of MSBR using an active psychoeducational (PE) control, 2) developing population-specific content and testing the efficacy of MSBR for parents of children with ASD, 3) utilizing a highly diverse, underserved community-based sample, 4) examining the mechanisms underlying observed treatment effects, and 5) employing multi-method longitudinal measurement from multiple sources in order to examine immediate and long-term treatment effects. Approach: The proposed study is a randomized controlled trial of 138 families of preschool-aged children with ASD. Parents will be randomized to MBSR or to a PE support group matched for clinical contact and dosage. Families will participate in laboratory assessments at baseline and immediately post-treatment, as well as at 6 months and 12 months post-treatment. Measures include standardized and validated parent and teacher questionnaires, gold-standard psychological assessments, and observational and interview ratings. Impact: Establishing an efficacious stress reduction intervention to target mechanisms linking parenting stress, negative parenting behaviors, and child externalizing problems will advance clinical science and optimize outcomes for children with ASD and their families. The proposal is based upon a history of substantial collaboration among research team members and involves faculty, graduate students, and undergraduates across two R15-eligible institutions that primarily serve underrepresented minority students, Loma Linda University (LLU) and California State University, Fullerton (CSUF). Consistent with R15 goals, the investigation will involve graduate and undergraduate students in every aspect of the research process. The study will enhance institutional training across two universities and will provide unparalleled clinical research experiences to underrepresented students in preparation for future careers in the behavioral sciences.